This invention relates to improvements in borescopes or similar elongated probes. The invention is more particularly concerned with a technique which facilitates measurement of a target object in a concealed area which is in the viewing field of the borescope or similar probe.
A borescope is generally characterized as an elongated flexible insertion tube with a viewing head at its distal or forward tip and a control section at its proximal end for controlling the bending at the distal end. There are also hard or rigid probes which have a rigid insertion tube and do not flex at the distal tip. A viewing device, such as a video monitor can be employed to process picture information received from the viewing head and produce a visible image of a target object within the viewing field of the viewing head.
Generally, there are two classes of probes, based on the viewing system. A fiber probe has an optical imager and carries the image on a fiber optic bundle to the viewing device. A video probe has a miniature television camera contained in the probe distal tip and a video signal is carried on a conduit to a video monitor to display the image of the target object. This invention is applicable to either class of probe, and of either rigid or flexible design.
A borescope is generally intended for visual inspection of an intricate mechanical assembly, such as a jet engine, a turbine, or a heat exchanger tube, where it would be difficult or impossible otherwise to view the internal parts of the assembly. The borescope of the flexible type needs to be insertable into narrow, tortuous passageways and must observe very delicate steering considerations. Thus, any appendage on the tip of the borescope should have flexible characteristics so that it will not degrade the ability of the borescope to pass through these difficult passageways.
At the present time, feeler wires can be inserted through an instrument channel of a probe to contact a target area. This technique is applicable only to the probes that have an instrument channel, and such probes are typically of a greater diameter than optimal for inspection of many types of equipment. Also, the feeler wire technique does not lend itself to quick, reliable, and automated measurement of objects.
Another approach to distance and size measurement is described in copending U.S. patent application Ser. No. 364,883, filed June 12, 1989, having a common assignee herewith. This technique involves projecting a shadow onto the target object, and then determining the distance to the target based on the position of the shadow.
The borescope has an illumination source that is off the axis of the video imager, and projects a shadow or other auxiliary image onto the object that is illuminated for viewing. The position of the shadow gives distance information, which in turn yields magnification of the image on the screen of the video display. A pre-calibrated object magnification and distance scale can be established for the geometry of the borescope optics and for the distance determined by shadow position. The scale can be placed on the video display for measuring the dimensions of objects in the field of view. Size information can be gleaned automatically, based on the number of pixels from a predetermined screen position to the shadow position, and on the number of pixels, horizontally and vertically, across the target object.
In the case of a turbine or jet engine, the target object can be a small crack or fracture on a stator vane or rotor blade. This measurement technique is useful for an assessment of whether the crack is greater than some critical limit at which replacement would be necessary. This technique can also be employed to follow dimensional changes in critical parts as the system ages.